The speed of scanning in a probe microscope can be increased by operating two or more cantilevers in parallel, such that data is acquired simultaneously from each probe. Parallel operation in scanning probe microscopy (SPM) is a challenge because multiple probe detection must be implemented as well as independent actuation systems for each cantilever. As a result, parallel SPM systems have in the past differed significantly from conventional SPM systems. For example, some systems have deployed cantilevers with integrated piezo-resistive sensors, and integrated zinc oxide Z-actuators (Quate et al Applied Physics Letters vol. 67 No 26 3918 (1995)). A major difficulty with such integrated systems is the complexity and corresponding cost of the sensors. The designs are also inflexible since changing a simple parameter such as the pitch or spring constant of the cantilevers also requires a redesign of the layout and costly fabrication. As a result, parallel SPM systems of this sort have not been widely used. There is therefore a need for a parallel probe microscope that is flexible in operation and configuration. Furthermore, such a system should incorporate a probe detection system and a probe actuation system that have at least the performance of conventional SPM, while retaining compatibility with cantilever probes widely used in SPM.
Conventional probe microscopes employ piezo-electric elements to scan the cantilever or specimen with nanometer level accuracy or better. However, such piezo-elements often have a limited speed of response due to their size and mechanical characteristics. Smaller elements which can be integrated into the cantilever can be employed for fast scanning applications but the required fabrication and electrical connection is a challenge.
Photothermal actuation has therefore been developed, in which an infra-red laser is focused onto a cantilever and used to induce photothermal bending of the cantilever for both z-actuation and resonant excitation (Yamashita et al, Rev. Sci. Instrum. Vol 78, 083702 (2007). This approach is powerful and flexible, and can achieve a rapid response time due to the small size and short thermal time constant of the micromechanical cantilever. However this approach has not been used for parallel probe control due to the increased number of optical components needed for alignment and focusing.
Conventional scanning probe microscopes typically rely on optical lever detection for sensing cantilever motion but these systems are impractical to operate with multiple probes as the alignment is time consuming and difficult to automate. Interferometer based motion sensing, both homodyne and heterodyne, has also been used in SPM, but no systems have been reported for parallel probe sensing. This is likely to be due to the complexity, difficulty of alignment and corresponding increase in optical components that is required when scaling interferometer systems.
Microcantilever biosensors have been demonstrated as sensitive tools for chemical and biological detection on chip. Microcantilevers biosensors can be used in two different modes of operation: static and dynamic. In the static mode, the binding of target molecules to the cantilever is detected as a result of the surface stress and cantilever bending they cause. In the dynamic mode the cantilever is actuated and its resonant frequency is determined. The binding of the molecules is detected due to the mass change and resulting resonant frequency shift. Resonant cantilevers immersed in liquid suffer from high damping losses and reduced sensitivity.
One of the major difficulties of cantilever sensing in either mode is the measurement of cantilever displacement. The most common method for this measurement is the optical lever approach. A focused laser beam is reflected off the cantilever surface, and captured by a PSD (Position Sensitive Detector). The cantilever displacement causes movement of the laser spot on the PSD and a change in its output voltage. This method is very sensitive, but it requires elaborate free-space optics with precise alignment of the laser beam to the device under test. Moreover, the PSD signal's relationship to the cantilever's displacement depends on the exact position of the laser spot on the cantilever. This relationship is unimportant for resonant frequency measurements, but it greatly impacts static mode operation. For example, a change in PSD output due to slight laser misalignment can be misinterpreted as cantilever bending. Since the alignment cannot be perfectly reproduced, the laser must be kept aligned to the cantilever throughout the static mode experiment. This complicates parallel measurements. If a cantilever array is exposed to a sample, the response of only one device can be captured. Custom-made arrays of lasers and PSDs for measuring several cantilevers in parallel have been demonstrated. However, this approach leads to greatly increased instrumentation complexity and difficulty of alignment. It is not feasible to increase the number of lasers much further, while the number of cantilevers on a chip can easily be in the hundreds or even thousands.
Another common method for measuring cantilever response involves the integration of on-chip displacement sensors. This approach not only allows multiple devices to be measured in parallel, but also simplifies the external measurement setup. The built-in sensors can be piezoresistive, piezoelectric, capacitive, transistor-based, or optical. Unfortunately, all of these technologies greatly increase the fabrication complexity and cost of the cantilevers, which should be simple, cheap and disposable.
There is therefore a need for a parallel cantilever array readout instrument suitable for both static or dynamic operation which is reasonably cheap, capable of automated operation, and compatible with a wide variety of environments, from gases to complex fluid physiological systems.